405 - permit loading just low levels of codebase

When I'm doing extensive surgery to the internals I want to avoid
loading higher levels; they aren't expected to work. But I don't want to
keep different levels in separate files just for that. And I definitely
don't want to put low-level stuff first. Now I can influence loading in
a cross-cutting manner by creating sections with numbers:

  (section 100
    ...code...)

And disabling them by running:

  $ ./anarki/arc 99 mu.arc.t

Currently we load all mu 'system software' in level 100, so running at
level 99 sidesteps them. Lower levels coming soon.

But most of the time we don't need to worry about levels, and the 'mu'
script lets us forget about them. Just run .mu files with:

  $ ./mu factorial.mu

To run tests:

  $ ./mu test mu.arc.t

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